1. Field of the Invention
The invention relates to a method for continuous catalytic hydrogenation, and in particular a method for preparing alicyclic carboxylic acids or their derivatives, in particular carboxylic esters, by selective hydrogenation of the corresponding aromatic carboxylic acid(s) (derivatives) in at least three series-connected reactors, at least the two first being operated in the loop operating mode.
2. Description of the Background
Alicyclic polycarboxylic esters, for example the esters of cyclohexane-1,2-dicarboxylic acid, are used as lubricating oil components and as aids in metal processing. In addition, they are used as plasticizers for polyolefins and PVC.
For plasticizing PVC, predominantly use is made of esters of phthalic acid, for example dibutyl, dioctyl, dinonyl or didecyl esters of phthalic acid. Since the use of these phthalates is increasingly under discussion in recent time as controversial, their use in plastics could be restricted. Alicyclic polycarboxylic esters, of which some are already described in the literature as plasticizers for plastics could then be used as suitable substitutes.
In most cases, the most economical route for preparing alicyclic polycarboxylic esters is nuclear hydrogenation of the corresponding aromatic polycarboxylic esters, for example of the abovementioned phthalates. Some methods are already known for this:
In U.S. Pat. Nos. 5,286,898 and 5,319,129, methods are described by which dimethyl terephthalate can be hydrogenated in the presence of supported Pd catalysts which are doped with Ni, Pt and/or Ru at temperatures greater than or equal to 140° C. and at a pressure between 50 and 170 bar to give the corresponding hexahydrodimethyl terephthalate.
U.S. Pat. No. 3,027,398 discloses the hydrogenation of dimethyl terephthalate in the presence of supported Ru catalysts at 110 to 140° C. and 35 to 105 bar.
In DE 28 23 165, aromatic carboxylic esters are hydrogenated to the corresponding alicyclic carboxylic esters in the presence of supported Ni, Ru, Rh and/or Pd catalysts at 70 to 250° C. and 30 to 200 bar. In this case use is made of a macroporous support having a mean pore size of 70 nm and a BET surface area of approximately 30 m2/g.
Further supported ruthenium catalysts for preparing alicyclic polycarboxylic esters by hydrogenating aromatic polycarboxylic esters are claimed in the patent documents WO 99/32427, WO 00/78704, DE 102 25 565.2 and DE 102 32 868.4.
WO 2004/046078 describes the hydrogenation of benzenepolycarboxylic acids or their derivatives in the presence of a catalyst which has the active catalyst metal applied on a support, the one or more materials having ordered mesopores.
The aromatic polycarboxylic esters are hydrogenated in U.S. Pat. No. 3,027,398 batchwise, in U.S. Pat. Nos. 5,286,898, 5,319,129, DE 28 23 165, WO 99/32427 and WO 00/78704 continuously in a tubular reactor without or with recirculation (loop operating mode) of the hydrogenation output.
In DE 102 32 868.4 and DE 102 25 565.2, the aromatic polycarboxylic esters are hydrogenated to the corresponding alicyclic polycarboxylic esters in two series-connected reactors, the first being operated in loop operating mode (partial recirculation of the reactor output) and the second being operated in straight through-flow passage. The first loop reactor can also be replaced by a plurality of small series- or parallel-connected loop reactors, these reactors having a shared circuit.
The technically known methods are not completely satisfactory with respect to the space-time yield and/or the selectivity. In addition, relatively large amounts of catalyst are required. It was an object of the present invention, therefore, to provide a hydrogenation method which can be carried out with as little catalyst as possible relative to the conversion rate to be achieved.
It has now been found that the required catalyst volume in the hydrogenation, in particular in the nuclear hydrogenation of aromatic carboxylic esters to the corresponding alicyclic carboxylic esters, can be minimized when the hydrogenation is carried out in at least three series-connected hydrogenation units, at least the two first hydrogenation units being operated in loop operating mode, i.e. with recirculation of a part of the respective hydrogenation output and with use of defined catalyst volumes in the individual hydrogenation units.